超快冷条件下Mn-Nb-B系低碳贝氏体高强钢组织与性能研究

采用Mn-Nb-B减量化成分设计的低碳贝氏体高强钢为研究对象,通过热模拟实验研究实验钢热变形行为和相变行为。结合中厚板生产线特点制定控制轧制与超快速冷却相结合生产工艺路线,充分利用超快速冷却条件下的细晶强化、析出强化等综合强化机制,实现综合力学性能优良的低成本高强工程机械用钢的试制和生产。产品屈服强度和抗拉强度分别达到678MPa和756MPa,伸长率A50为33%,-20℃低温冲击达到261J。产品显微组织由粒状贝氏体、针状铁素体和板条贝氏体组成,基体组织内弥散分布着细小的点状、粒状M/A岛和均匀细小的(Nb,Ti)(C,N)析出粒子以及大量位错组织。     

形变热处理对T91钢组织和性能的影响

用热模拟机模拟T91铁素体耐热钢的形变热处理工艺,在T91钢的奥氏体未再结晶区(650-850℃)对其进行不同程度的变形,研究了直接淬火对T91钢的组织形成和力学性能的影响.结果表明:与传统热处理相比,形变热处理不仅使T91钢的组织明显细化,而且生成更多的MX型纳米级碳氮化物颗粒.通过拉伸实验表明:形变热处理工艺可以明显提高T91钢的强度,进而达到提高其使用温度的目的.     

热处理工艺对新型轻质奥氏体耐磨钢的组织与力学性能的影响

以新型轻质高锰、高铝的奥氏体耐磨钢为研究对象,利用XRD,OM,SEM,EDS观察显微组织和析出物,研究不同的热处理工艺对新型钢种的组织与力学性能影响。结果表明:该新型轻质奥氏体耐磨钢的最佳优化热处理工艺为1050℃保温1h水韧,550℃时效2h,空冷。在最佳热处理工艺条件下奥氏体基体内弥散析出细小的钙钛矿结构(Fe,Mn)3AlC的κ-碳化物颗粒,不仅强化了奥氏体基体,其力学性能也得到明显改善;最优工艺处理后实验钢的硬度、强度、冲击韧度达到了最佳匹配,其抗拉强度为825MPa,屈服强度为574MPa,冲击韧度值为156J/cm2(V型缺口),硬度为271HB;与只进行水韧处理相比实验钢的屈服强度提高40.0%,硬度提高32.2%。     

Mn含量对热轧超低碳钛低合金钢组织与力学性能的影响

实验钢在传统C-Mn钢的基础上添加低合金元素Ti,通过调整钢中Mn元素含量,同时采用简便的控制轧制与控制冷却工艺,获得了良好的组织形态及纳米尺度析出物,从而在保证优良延伸性能的前提下大幅度提高了钢板的强度,显著降低了钢材成本。使用金相显微镜(OM)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)对微观组织进行观察。结果表明:当实验钢Mn含量从1.05%(质量分数,下同)提高至1.5%,平均晶粒尺寸从6.4μm细化至5.2μm;基体中纳米尺度TiC的析出量明显增加;屈服强度、抗拉强度和断后伸长率分别提高了56.7,42.2MPa和1.2%,达到了558.7,662.2MPa和22.4%。     

N含量对Cr-Mo-V系超低碳贝氏体钢组织性能和析出行为的影响

通过光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和能谱仪(EDS)等实验方法,研究了三种不同N含量的超低碳贝氏体钢的显微组织和析出相的成分、尺寸、形貌以及分布等特征。结果表明:低氮含量的钢组织为粒状贝氏体,高氮含量的钢组织为粒状贝氏体+少量的针状铁素体。当实验钢中V/N比为3.4时,通过细晶强化和沉淀强化综合作用,可以使材料的屈服强度和抗拉强度分别增加231MPa和95MPa。与氮含量低的钢相比,高氮含量的钢具有更细小的贝氏体铁素体板条亚结构,且析出相尺寸减小,体积分数增加。基体中存在两种尺寸的纳米级析出相:一种尺寸在10～15nm之间,为V(C,N)析出相,弥散分布在贝氏体板条内部;另一种是含有Cr和V尺寸在10nm以下,具有面心立方结构的(V,Cr)(C,N)复合析出相。     

N含量对Cr-Mo-V系超低碳贝氏体钢组织性能和析出行为的影响EI北大核心CSCD

通过光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和能谱仪(EDS)等实验方法,研究了三种不同N含量的超低碳贝氏体钢的显微组织和析出相的成分、尺寸、形貌以及分布等特征。结果表明:低氮含量的钢组织为粒状贝氏体,高氮含量的钢组织为粒状贝氏体+少量的针状铁素体。当实验钢中V/N比为3.4时,通过细晶强化和沉淀强化综合作用,可以使材料的屈服强度和抗拉强度分别增加231MPa和95MPa。与氮含量低的钢相比,高氮含量的钢具有更细小的贝氏体铁素体板条亚结构,且析出相尺寸减小,体积分数增加。基体中存在两种尺寸的纳米级析出相:一种尺寸在10～15nm之间,为V(C,N)析出相,弥散分布在贝氏体板条内部;另一种是含有Cr和V尺寸在10nm以下,具有面心立方结构的(V,Cr)(C,N)复合析出相。     

热处理温度对新型马氏体时效不锈钢微观组织和性能的影响

针对一种以Al作为主要强化元素的新型马氏体时效不锈钢,通过力学性能测试、光学显微镜观察和透射电子显微分析方法,研究不同的热处理温度对实验钢力学性能和微观组织的影响。结果表明:该实验钢的抗拉强度最高可达1876MPa,屈服强度可达1762MPa,具有良好的强韧性配合。固溶处理后形成了具有高密度位错的细小板条马氏体组织,在时效过程中,马氏体基体上弥散析出的NiAl相使其强度得到大幅度的提升。随着时效温度的提高,NiAl析出相颗粒逐渐长大粗化,从而使强度在到达峰值后迅速下降,出现了过时效现象。实验钢经过820℃固溶+(-70℃)冷处理+540℃时效处理后可获得良好的综合力学性能。     

α区形变对微合金钢中析出相和强度的影响

采用Gleeble热模拟实验机对含Nb、Mo微合金钢进行了铁素体(α)区形变+等温工艺实验,研究了微合金钢中析出相的变化及其对强度的影响。利用透射电镜对不同工艺所制备试样中析出相的形貌进行观察和分析,纳米级析出相弥散分布于铁素体基体之中;并对析出相的尺寸进行统计分析,结果表明,相较于等温工艺,形变+等温工艺可以明显细化析出相的尺寸,且在600℃下获得的析出相尺寸更为细小,平均尺寸为2.77 nm。利用Thermo Calc数据库计算平衡条件下600和700℃时(Nb_1-xMo_x)C中的x值和析出相的体积分数,结合实验中统计的析出相尺寸,分析了不完全析出条件下(Nb_1-xMo_x)C析出相对屈服强度的贡献。结果表明,在600℃进行形变+等温工艺,尽管析出相的体积分数不高,但因(Nb_1-xMo_x)C的尺寸更小,其提供的屈服强度增量更高。     

过冷对共析钢动态相变和组织演变的影响

将共析钢在600-700℃的过冷奥氏体状态下进行单轴热压缩,获得了亚微米级别等轴铁素体 纳米级别弥散分布球化渗碳体的复相组织,其组织演变经历动态相变、片层渗碳体球化、铁素体动态再结晶和纳米级别渗碳体颗粒析出等过程.随着形变温度的降低,过冷度增加,相变动力学过程加快.片层渗碳体的球化程度由球化时间和球化速度控制,形变温度升高使渗碳体球化速度加快,但是相变开始时间及动力学延迟使得用于渗碳体球化和熟化的时间相应缩短,导致球化程度降低.铁素体再结晶和等轴化过程则主要受位错迁移、渗碳体颗粒钉扎的影响,形变温度升高导致较高的等轴化发展速度.     

Nb微合金化对Q500MPa级热轧H型钢组织和性能的影响

采用纳米压痕试验、电子背散射衍射、透射电镜等研究Nb元素对热轧H型钢组织和性能的影响。结果表明,H型钢中添加0.025%(质量分数,下同)的Nb,在轧制和冷却工艺相同的情况下,屈服强度和-20℃低温冲击功分别为552 MPa、102 J,相比不含Nb的试样分别提升了17.7%和20.0%,且晶粒尺寸从5μm细化至4μm。通过热力学计算发现铁素体基体内形成的复合析出物(Nb_(1-x)V_x)C与基体的界面能比NbC与基体的界面能更小且形核驱动力更大,因此更易促进Nb以复合析出物形式析出,且纳米析出物平均尺寸从16.3 nm减小至11.46 nm,体积分数从0.053%提高至0.108%,析出强化由55 MPa增加至102 MPa,铁素体硬度由247HV提升至279HV,添加Nb元素后析出强化贡献增量较细晶强化增量高18 MPa。     

Combined effects of high-temperature rolling and ultrafast cooling on the mechanical properties of low-carbon steel

The role of ultrafast cooling (UFC) on the grain refinement of ferrite, the precipitation behavior of cementite particles and the mechanical properties of a mild steel (Q235 grade) was evaluated by applying laminar cooling and UFC and varying the finish cooling temperature ranges during UFC after hot rolling. While UFC refined the ferrite grains, it accumulated the degeneration of pearlite, resulting in complete disappearance of the laminar pearlite at relatively low finish cooling temperatures. The minimum mean size of spheroidized cementite particles reached ~110?nm. Meanwhile, the enhancement of UFC on tensile strengths of mild steels mainly resulted from the grain refinement of ferrite and the precipitation strengthening of cementite particles; however, the contribution varied with the finish cooling temperature of UFC. A modified Ashby–Orowan model was also used for evaluating the yield strength increment of medium plates. This work will provide a theoretical basis for the diversity control of microstructure and for developing stronger and tougher mild steels by introducing UFC technology after high-temperature rolling.     

Effects of carbon and niobium on microstructure and properties for Ti bearing steels

The objective of the study described here is to elucidate the effect of carbon and niobium on the microstructure, precipitation behaviour, and mechanical properties of 0·09C–0·11Ti (%) steel and 0·05C–0·025Nb–0·11Ti (%) steel under ultra fast cooling condition. The strengthening mechanisms are also discussed. The ferrite grains size and the size of precipitates in Ti and Nb–Ti steels were measured respectively. The mechanical properties obtained in Ti steel were similar to Nb–Ti steel with yield stress >700 MPa, elongation >20%, and good low temperature impact toughness. The study underscores that addition of higher carbon content by 0·04% under controlled rolling and ultra fast cooling conditions, we can achieve similar strength in the absence of micro-alloying element, niobium.     

Copper bearing microalloyed ultrahigh strength steel on a pilot scale: Microstructure and properties

In the present study, copper bearing low carbon microalloyed ultrahigh strength steel has been produced on a pilot scale. Transformation of the aforesaid steel during continuous cooling has been evaluated. The steel sample has been thermomechanically processed followed by either air cooling or water quenching. Variation in microstructure and mechanical properties at different finish rolling temperatures has been studied. A mixture of granular bainite, bainitic ferrite and precipitation of nano-sized (Ti, Nb)C particles is the characteristic microstructural feature of air cooled steel. On the other hand, predominantly lath martensitic structure along with the similar type of microalloying precipitates of air cooled steels and Cu precipitates are obtained in case of water quenched steel. The best combination of strength (1364-1403 MPa) and ductility (11-14%) has been achieved for the selected range of finish rolling temperature of water quenched steel.     

Effects of precipitates on strength and toughness of vanadium structural steels

Abstract

The effects of precipitates formed by the addition of vanadium, nitrogen, and aluminium on the strength and toughness in 0·15%C steels were studied by cooling suitable steels at varying rates from temperatures giving a constant austenite grain size. Yield strength increments resulting from precipitation were derived from the experimental results. Similar increments were produced by vanadium carbide (VC) and vanadium carbonitride (V(C,N)) and in each case a reduction in cooling rate resulted in reduced strengthening because of increased particle coarsening. In the presence of aluminium rather less strengthening was obtained from V(C,N) precipitates. No strengthening resulted from the presence of aluminium nitride (AIN) particles. When the carbon content of the steel is greatly in excess of the stoichiometric quantity, the carbonitride formed is relatively low in nitrogen and the yield strength increments caused by low nitrogen V(C,N) and VC are similar. Impact transition temperature increments resulting from precipitation were also derived from the experimental data. These showed that V(C,N) precipitates are less detrimental to transition temperatures than are VC precipitates giving comparable yield strength increments. Shelf energies were also reduced to a greater extent by VC than by V(C,N). The presence of AIN particles had little effect on shelf energy, but had a detrimental effect on transition temperature. The high nitrogen V(C,N) precipitates are less detrimental to both impact transition temperatures and upper shelf energies of the steels than are low nitrogen V(C,N) or VC precipitates. Removal of soluble nitrogen in the form of nitrides has been confirmed as a means of improving the toughness of steels. Aluminium has been found to be more effective than vanadium in the removal of soluble nitrogen.

MST/935     

基于纳米铁碳析出物的钢综合强化机理

简要叙述了关于钢强化机理的研究现状,用化学相分析+X射线小角散射、RTO方法及高分辨透射电镜对薄板坯连铸连轧钛微合金化高强耐候钢中纳米粒子的属性进行了综合分析。发现钛微合金化高强耐候钢中尺寸<36 nm的粒子,除纳米TiC以外,还存在大量的纳米Fe3C,其体积分数为同尺寸TiC体积分数的4.4倍,析出强化作用比纳米TiC粒子大,不可忽略;提出了钢的综合强化机理,指出对不同种类、不同尺寸的纳米析出粒子,应分别根据位错切割和位错绕过机理计算出析出强化贡献,然后与固溶强化和细晶强化贡献加和,求得钢的屈服强度;讨论了细晶强化与位错强化不能加和的原因以及相变对钢强度影响的表现形式,对钛微合金化高强耐候钢屈服强度的理论计算与生产结果相符,实验钢σs=630～676 MPa,实际σs=630～680 MPa。简述了钢综合强化机理的应用情况,指出了有待进一步研究的科学问题。     

Mathematical Modelling of Carbonitride Precipitation during Hot Working in Nb Microalloyed Steels

Based on thermodynamics and kinetics, precipitation behavior of microalloyed steels was analyzed. Deformation greatly promotes isothermal carbonitride precipitation and makes C-curve shift leftwards. The position and shape of C-curve also depend on the content of Nb and N. C-curve shifts leftwards a little when N content increases and the nose temperature is raised with increasing Nb content. Deformation shortened precipitation start time during continuous cooling, raised precipitation start temperature, accelerated precipitation kinetics of carbonitrides. With decreasing the finishing temperature and coiling temperature, the precipitates volume fraction increases and strength increment is raised during hot rolling. The simulated results are in agreement with experiment results.     

新一代舰船用钢制备技术的现状与发展展望

综述了美国舰船用钢的发展历程,总结了其成分体系、工艺技术及典型应用,分析了新一代舰船用钢的发展趋势。介绍了我国舰船用钢的主要发展历程,指出了我国舰船用钢与国际先进水平的主要差距。对于新一代极低碳复合析出强化型高强韧钢进行了探索性研究。结果发现,采用极低碳成分并结合控制轧制工艺技术,可以获得强度和韧性的良好匹配。对实验钢的显微组织和析出相进行了检测分析,对强韧化机制进行了初步的阐述。最后,概述了我国高强韧钢生产的关键技术及装备基础。我国自主开发的新一代TMCP工艺技术和装备已达到国际先进水平,表明我国新一代舰船用钢由“跟踪”向“自主研发”转变已经具备了坚实的装备和工艺基础。     

冷却速度和卷取温度对Nb-V—Ti复合微合金带钢铁素体中析出的影响

采用一种工业生产的Nb-V-Ti复合微合金钢在Gleeble热模拟试验机上研究了精轧后冷却速度（1～30℃／s）和卷取温度（650～550℃）对铁素体中析出的影响。研究结果表明,在1～5℃／s的冷却速度下观察到了10～20nm的相间析出。在650℃和600℃卷取时,随着轧后冷却速度的增加,10nm以下的一般析出数量增多而尺寸减小;而在550℃卷取时,当冷却速度为30℃／s时10nm以下的析出数量急剧减少,这可能与较慢析出动力学有关。在相同的轧后冷却速度下,当卷取温度降低时,一般析出的体积分数增加而尺寸减小,这与析出的热力学驱动力增加有关。     

Strain aging and carbide precipitation in aluminium-killed steels

Abstract

The effect of 0·003–0·050 wt-%C additions on strain aging and carbide precipitation in batch-annealed aluminium–killed steels has been investigated. The low–carbon steels exhibited unusual strain–aging characteristics: steels containing 0·020–0·040 wt–%C strain aged at 160 but not at 50°C, whereas those containing 0.007–0·020 wt–%C aged at both temperatures. This behaviour is due to the presence of fine carbide particles, which form as a result of the supersaturation of carbon in the matrix during cooling and which raise the carbon solubility by the Thomson-Freundlich effect. Expressions are derived which give the carbon supersaturation as a function of temperature during cooling, and of the precipitation temperature of each class of carbide. Carbide precipitation alternates between the nucleation of new particles and the growth of existing ones, depending upon the particle spacing and the cooling rate. Strain aging in the 0·05 wt–%C steel is negligible up to 225°C, but then rises to a peak at 325°C, which has been attributed to effects of partition in the dislocation matrix.

MST/141     

Effect of Nb-Mo additions on precipitation behaviour in V microalloyed TRIP-assisted steels

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation-induced plasticity-assisted steels with an acicular/bainitic ferrite matrix were investigated by a heat treatment up to the austenite formation range. It was found that during the heating stage the acicular/bainitic ferrite matrix resisted recrystallisation, while cementite and martensite were decomposed and austenite was formed in the acicular/bainitic ferrite. Both Nb-V-Mo and V containing steels after the heat treatment showed a microstructure consisting of a polygonal ferrite matrix with small islands of pearlite. During these transformations, the microscopy observations showed that 0.04 wt% Nb and 0.08 wt% Mo additions to the 0.16 wt% V microalloyed steel considerably reduced the growth-coarsening of microalloy precipitates.